Evidence for genetic incompatibilities associated with post-zygotic reproductive isolation in the human fungal pathogen Cryptococcus neoformans

Hybridization is a potent mechanism for generating unique strains with broad host ranges and increased virulence in fungal pathogens. In the opportunistic basidiomycete pathogen Cryptococcus neoformans, intervarietal hybrids are commonly found infecting patients. The two parental varieties C. neoformans var. grubii and C. neoformans var. neoformans mate readily under laboratory conditions, but the hybrid basidiospores have germination rates about four times lower than those from intravarietal crosses. Here, we used microdissection to collect basidiospores from a hybrid cross and analysed the genotypes of germinated basidiospores to identify potentially antagonistic allelic combinations between loci that impact basidiospore germination. Our analyses showed clear evidence for Bateson-Dobzhansky-Muller (BDM) incompatibility affecting basidiospore viability. Antagonistic combinations of alleles from both two loci and three loci were found. Interestingly, most of the hybrid progeny showed segregation distortion in favour of the alleles from var. neoformans, consistent with large-scale epistatic interactions among loci affecting basidiospore viability. Our study presents the first evidence of BDM incompatibility between nuclear genes affecting post-zygotic reproductive isolation in this model basidiomycete yeast.